To meet the request for miniaturization and high integration of electric devices and electronic devices in recent years, the pitch of circuit patterns on a printed circuit board has been made smaller. A circuit pattern having a fine pitch (distance between neighboring patterns) of not more than about 75 μm is formed by an additive method, a semi-additive method and the like.
According to the additive method, for example, a resist pattern is formed on an insulating substrate, a conductive layer made of copper is formed in an area other than the resist pattern by plating, and the resist pattern is removed. In this way, a circuit pattern is formed in an area other than the resist pattern.
According to the semi-additive method, a metal backing layer of Cr and the like is first formed on an insulating substrate by plating, vapor-deposition and the like, and a resist pattern is formed on the metal backing layer. Subsequently, Cu layer is formed on the metal backing layer in the area other than the resist pattern by plating, and the resist pattern is removed to form a circuit pattern. Then, using the circuit pattern as a mask, the metal backing layer is etched. In this way, a circuit pattern is formed in the area other than the resist pattern. In the printed circuit board produced by the semi-additive method, elution of a metal ion constituting the circuit pattern into the insulating substrate, which is what is called ion migration, can be prevented, because the circuit pattern is formed on the insulating substrate via the metal backing layer of Cr layer and the like.
In the production of a printed circuit board according to the additive method or the semi-additive method, variation in the thickness of circuit pattern has become a problem. That is, a pattern present in the central area of a printed circuit board where a number of circuit patterns are disposed, and a pattern present in the outer peripheral area of the printed circuit board surrounded by an area free of a circuit pattern show different current densities during plating. As a result, despite the fact that they have been formed by the same process, the pattern present in the central area and that present in the outer peripheral area have different thicknesses. To solve this problem, a dummy pattern has been conventionally formed on the outside of the circuit pattern (JP-A-2001-101637).
FIG. 9(a)–FIG. 9(c) show concrete example of such dummy pattern.
FIG. 9(a) shows an embodiment wherein a solid dummy pattern is disposed near a circuit pattern, FIG. 9(b) shows an embodiment wherein a circuit pattern is surrounded by a solid dummy pattern, and FIG. 9(c) shows an embodiment wherein a dummy pattern having the same shape as a circuit pattern is arranged near the circuit pattern.
However, the dummy pattern is inherently unnecessary for the printed circuit board and needs to be removed ultimately. When a dummy pattern is simultaneously removed (dissolved) along with an insulating substrate in an etching step for the insulating substrate, without particularly applying a step for exclusively removing the dummy pattern, the dummy pattern is not completely dissolved and remains as a residue. In the above-mentioned case, therefore, after forming a circuit pattern (dummy pattern), the dummy pattern is removed by etching and the like. In other words, forming a dummy pattern necessitates a removal step of the dummy pattern, which in turn makes the production cost of a printed circuit board expensive.